1.6 Math and Measurement for Medical Assistants
Key Takeaways
- The metric system is the standard in healthcare: length (meters/centimeters), mass (grams/kilograms), volume (liters/milliliters), and temperature (Celsius)
- Key conversions: 1 kg = 2.2 lbs, 1 inch = 2.54 cm, 1 tsp = 5 mL, 1 tbsp = 15 mL, 1 oz = 30 mL, 1 cup = 240 mL
- Temperature conversions: °F to °C = (°F - 32) × 5/9; °C to °F = (°C × 9/5) + 32
- Medication math uses the formula D/A × Q = Amount (Desired dose ÷ Available dose × Quantity)
- Intake and output (I&O) measures all fluids consumed and all fluids excreted to assess fluid balance
- Military time (24-hour clock) is used in healthcare documentation: 1:00 PM = 1300, midnight = 0000, noon = 1200
Math and Measurement for Medical Assistants
Accurate math and measurement skills are essential for safe clinical practice. Medical assistants must be comfortable with metric conversions, medication calculations, and documentation standards.
Metric System Basics
| Measurement | Base Unit | Common Clinical Units |
|---|---|---|
| Length | Meter (m) | Centimeters (cm), millimeters (mm) |
| Mass/Weight | Gram (g) | Kilograms (kg), milligrams (mg), micrograms (mcg) |
| Volume | Liter (L) | Milliliters (mL), cubic centimeters (cc) — Note: 1 mL = 1 cc |
| Temperature | Celsius (°C) | Also Fahrenheit (°F) in some clinical settings |
Metric Prefixes:
- kilo- (k) = 1,000 (1 kg = 1,000 g)
- centi- (c) = 0.01 (1 cm = 0.01 m)
- milli- (m) = 0.001 (1 mg = 0.001 g; 1 mL = 0.001 L)
- micro- (mc/μ) = 0.000001 (1 mcg = 0.001 mg)
Essential Conversions
| Conversion | Value |
|---|---|
| 1 kg | 2.2 pounds (lbs) |
| 1 inch | 2.54 centimeters |
| 1 teaspoon (tsp) | 5 mL |
| 1 tablespoon (tbsp) | 15 mL |
| 1 ounce (oz) | 30 mL |
| 1 cup | 240 mL (8 oz) |
| 1 pint | 480 mL (16 oz) |
| 1 quart | 960 mL (32 oz) |
| 1 liter | 1,000 mL |
Weight Conversion Examples:
- Patient weighs 154 lbs → 154 ÷ 2.2 = 70 kg
- Patient weighs 80 kg → 80 × 2.2 = 176 lbs
Temperature Conversions
- °F to °C: (°F - 32) × 5/9 = °C
- Example: 98.6°F → (98.6 - 32) × 5/9 = 37°C
- °C to °F: (°C × 9/5) + 32 = °F
- Example: 39°C → (39 × 9/5) + 32 = 102.2°F
Military Time (24-Hour Clock)
Healthcare documentation uses 24-hour (military) time to prevent AM/PM confusion:
| Standard Time | Military Time |
|---|---|
| 12:00 AM (midnight) | 0000 |
| 1:00 AM | 0100 |
| 6:00 AM | 0600 |
| 12:00 PM (noon) | 1200 |
| 1:00 PM | 1300 |
| 3:00 PM | 1500 |
| 6:00 PM | 1800 |
| 9:00 PM | 2100 |
| 11:59 PM | 2359 |
Conversion rules:
- For times 1:00 AM to 12:59 PM: Same number (add leading zero if needed)
- For times 1:00 PM to 11:59 PM: Add 12 to the hour
- To convert from military time back: If ≥1300, subtract 12
Intake and Output (I&O)
I&O measures all fluids consumed (intake) and all fluids excreted (output) over a defined period:
Intake includes: Oral fluids, IV fluids, tube feedings, ice chips (counted at half volume) Output includes: Urine, vomit, diarrhea, wound drainage, blood loss
Normal adult daily fluid balance: Intake ≈ Output (approximately 2,000-2,500 mL/day)
A patient weighs 176 pounds. What is their weight in kilograms?
What is 3:30 PM in military (24-hour) time?
How many milliliters (mL) are in 2 tablespoons of liquid medication?